1. Field of the Invention
The present invention relates to a process for purifying methyl methacrylate, and more particularly to a process for purifying the same by distilling a mixture containing methanol, methyl methacrylate, a small amount of methyl isobutyrate and optionally water to remove the methanol and the methyl isobutyrate, or to remove the methanol, the methyl isobutyrate and the water.
2. Description of the Prior Art
As the process for producing methyl methacrylate, there have been known the acetonecyanhydrin process and the isobutylene (or t-butanol) oxidation process. The latter process includes a process comprising oxidizing isobutylene or t-butanol into methacrolein, further oxidizing the same into methacrylic acid, and then reacting the same with methanol into methyl methacrylate, and a process comprising reacting in one step methacrolein with oxygen and methanol to produce methyl methacrylate.
The reaction products obtained in the preparation of methyl methacrylate by esterifying methacrylic acid with a large excess of methanol or by oxidizing methacrolein with oxygen in methanol contain methanol in an amount as large as 20 to 85% by weight and form a homogeneous phase in many cases. Although the excess methanol is usually separated, recovered and reused in the reaction, the extraction of methanol with water from said homogenous mixture requires a large amount of water, resulting in economical disadvantages such as an increased amount of materials to be treated, an increased utility cost for recovering methanol from the water-methanol solutions, and the like. In addition, the above feed mixture forms a complicated azeotropic mixture, so that it is difficult to separate and purify methyl methacrylate from said feed mixture by mere distillation.
On the other hand, a method of the extraction of methyl methacrylate in the presence of a hydrocarbon and water from the above feed mixture is known as another method for isolating methyl methacrylate from the feed mixture. This method is, however, troublesome because it requires the recovery of methyl methacrylate from the hydrocarbon and of methanol from the water-methanol solution.
Moreover, the reaction mixtures obtained by these processes for preparing methyl methacrylate contain a variety of impurities originated from the raw materials and from side reactions. In particular, when methyl isobutyrate is contained as an impurity, the separation thereof is markedly difficult even by means of distillation, extraction, or the like because the boiling point of methyl isobutyrate (92.3.degree. C.) is very close to that of the objective methyl methacrylate (100.3.degree. C.) and the difference between the two in solubilities in water and other solvents is also small. Therefore, the conventional process requires a complicated process for the separation and also requires a utility such as much steam or the like for the separation. On the other hand, if methyl isobutyrate is present in the methyl methacrylate, it follows that in the polymeric products obtained by polymerizing the methyl methacrylate, there remains the methyl isobutyrate which does not participate in the polymerization, and this results in a deterioration of the quality of the polymeric product. Therefore, it is essential to remove this impurity even though the step therefor is complicated.
As an example of the known separation process, Japanese Patent Application Kokai (Laid-Open) No. 146,418/76 proposes a method which comprises distilling a crude methacrylate containing an isobutyrate in the presence of a small amount of water, separating the distillate into the organic phase and the aqueous phase, and further distilling said organic phase in the presence of water to concentrate and remove the isobutyrate. However, according to this proposal, it is necessary to form two liquid phases of water and organic matter at the top of a distillation column, and therefore, when a large amount of methanol or the like is present a single uniform phase is formed. Therefore, only a minute amount of methanol is allowed to co-exist, and these water-soluble organic matters must be previously removed to obtain crude methacrylate free from the water-soluble organic matters.
However, as mentioned above, the reaction product obtained in the preparation of methyl methacrylate by esterifying methacrylic acid with an excess of methanol or by oxidizing methacrolein with oxygen in methanol contains methanol in an amount as large as 20 to 85% by weight, and the content of methyl isobutyrate in the reaction product usually comes to 0.05 to 2% by weight based on the weight of methyl methacrylate. When it is intended to separate methyl isobutyrate by distillation from said feed mixture the methyl isobutyrate and the methyl methacrylate both form azeotropic mixtures with methanol and water, and hence, it is substantially impossible to remove the methyl isobutyrate from the methyl methacrylate by distillation. As a method of separating and purifying methyl methacrylate from said feed mixture, there is also known a method of extracting methyl methacrylate with a hydrocarbon. However, the methyl isobutyrate is difficult to remove from methyl methacrylate because the extraction of the methyl isobutyrate with a hydrocarbon is easy equally to or easier than that of the methyl methacrylate.